The invention resides in a press drive for a press as well as in a method for operating the press. The press drive includes an elbow lever drive. The elbow lever drive is driven by an eccentric drive and serves for coupling the eccentric drive with the plunger of the press so as to move it in the stroke direction.
Presses with elbow lever drives are generally known. DE 10 2005 001 878 B3 discloses a press drive with an elbow lever drive, wherein an auxiliary drive is assigned to the plunger of the press. This auxiliary drive is, in particular, intended to ensure a sufficient plunger force in certain angular position areas of the levers of the elbow lever drive.
DE 10 2007 002 715 A1 discloses an elbow lever drive with two elbow lever drive arrangements which can be operated via a common linear drive which activates the elbow lever joints.
DE 21 27 289 A discloses an adjustable elbow lever drive. A main eccentric drives a main connecting rod which forces the first lever of the elbow lever drive, which first lever is connected to the plunger via a second lever. An auxiliary eccentric acts via an auxiliary connecting rod, one arm of the two-arm lever. The other arm of the two-arm lever is coupled to the elbow joint. The connecting points of the auxiliary connecting rod as well as the drive rod between the two-arm lever and the elbow joint are adjustable. In this way it becomes possible to adjust the impact speed of the plunger on the workpiece, the travel distance of the plunger stroke, the stroke length and the position of the lower reversal point.
Another press with an elbow lever drive is described in DE 198 46 951 A1. The first lever of the elbow lever drive is supported on the press frame whereas the other lever is connected to the plunger. The two levers are interconnected by way of a triangular link so that the first lever and the second lever are connected to the triangular link so as to be pivotable about spaced pivot axes. The triangular link is furthermore connected to an eccentric drive. The length of the arm, the triangular link which is connected to the eccentric drive, can be changed. When the elbow lever drive pivots through its stretched position, the plunger is moved, because of the kinematics of the arrangement, twice through a lower reversal point shortly after one another. The position of these two lower reversal points differs with respect to a reference point on the press frame. If the elbow lever drive does not pivot through its stretched position, a manual about sine-shaped plunger position path is obtained.
This arrangement has the disadvantage that the positions of the low reversal points are different when the elbow drive is moved through its lower stretched position. On the other hand, a changing connecting rod length is in many cases undesirable. A connecting rod of finite length causes at its drive end always a distance-time course which is not identical with a sine or cosine shape. These deviations from a sine or cosine shaped course of movement change if the length of the connecting rod changes. The shorter the connecting rod the greater is the deviation from the sine- or respectively cosine form. Furthermore, the length change of the connecting rod, if to be provided by a control drive, is expensive in its design and increases the moving mass noticeably.
Based on this state of the art, it can be considered to be the object of the invention to provide a press drive and a method for operating such a press drive which ensures a very simple and cost-effective design and which provides at the same time for high flexibility in the use of the press.